In fabricating very large scale integration (VLSI) and ultra large scale integration (ULSI) circuits it is increasingly important to provide maximum areal density for the devices from which the circuits are formed. As is well known to those having skill in the art, in a VLSI or ULSI regime, millions of active devices may be formed on a single integrated circuit chip. These active devices must be electrically isolated from one another. In providing isolation regions between devices, it is important that the area devoted to isolation regions be minimized so that the device area is maximized.
Recessed oxide (ROX) isolation techniques are widely used for device isolation in silicon semiconductor substrates. In a recessed oxide isolation process, a silicon nitride or silicon nitride-silicon dioxide composite structure, or other mask is used to pattern the surface of the silicon semiconductor substrate and thereby define exposed regions thereon. The silicon substrate is then thermally oxidized at high temperatures of about 1000.degree. C., in order to convert the exposed regions to silicon dioxide, which form the recessed oxide isolation regions.
Unfortunately, it has been found that when recessed oxide is formed by thermally oxidizing exposed portions of a silicon substrate, the oxidation occurs laterally as well as vertically. This lateral encroachment of the oxide reduces the area available for device fabrication. Since the cross section of the laterally encroaching oxide looks like a bird's beak, the lateral encroachment phenomena has been referred to as the "bird's beak" phenomena.
As the device density of integrated circuits has increased, bird's beak has become more of a problem. For example, if a device is 40 .mu.m wide and the bird's beak encroaches by 2 82 m, only a slight reduction in the device area to 36 .mu.m is produced. However, if a VLSI or ULSI device is less than 4 .mu.m wide, the bird's beak encroachment can completely eliminate the device area. Accordingly, the art has concentrated on techniques for minimizing bird's beak.
Many bird's beak reduction techniques use a silicon nitride mask layer on the silicon semiconductor substrate, to prevent thermal oxidation of the semiconductor substrate lying thereunder. See for example U.S. Pat. Nos. 4,662,312 to Aoki, 4,775,644 to Szeto, 4,818,235 to Chao, 4,855,258 to Allman et al. and 4,686,762 to Chai et al. However, it has been found that the silicon nitride mask does not completely eliminate the bird's beak. Moreover, silicon nitride produces compatibility problems with silicon substrates. The silicon nitride mask must also be removed before devices may be formed in the substrate.
Other techniques for preventing bird's beak use polycrystalline silicon masks. Polycrystalline silicon, often referred to as "polysilicon" or simply "poly", is a well characterized material and is highly compatible with standard silicon processing steps. As such, polycrystalline silicon is widely used in semiconductor device fabrication, for example for device contacts. However, it is well known that polycrystalline silicon alone cannot be employed as a mask against thermal oxidation in the recessed oxide isolation process, because polycrystalline silicon oxidizes like monocrystalline silicon during thermal oxidation. Accordingly, if polycrystalline silicon is used it must be encapsulated to prevent oxidation. See, for example, U.S. Pat. Nos. 4,746,625 to Morita et al. and 4,541,167 to Havemann et al. ROI processes using polycrystalline silicon are typically complex, requiring many extra fabrication steps, and do not wholly eliminate the bird's beak.
Thermal oxidation also creates other problems in semiconductor device fabrication because it is a high temperature process. Diffusion of impurities is enhanced at high temperatures making it difficult to form pure devices with controlled impurity levels. Moreover, high temperature processing also produces device variability and reduces process uniformity. For example, in fabricating field effect transistor devices, nonuniform threshold voltage sensitivity as a function of substrate voltage is obtained as a result of high temperature processing.
An attempt to eliminate bird's beak using low temperature processing steps is described in publications entitled "Plasma Oxide FET Devices" and "The Formation of SiO.sub.2 in an RF Generated Oxygen Plasma", published in Volume 128, Number 11 of the Journal of the Electrochemical Society, November 1981, pages 2424-2428 and 2466-2472, respectively, by A. K. Ray and A. Reisman. In these publications, a low temperature plasma oxidation process is described wherein a radio frequency (RF) generated oxygen plasma is produced at pressures above 10 mtorr and at nominal temperatures of about 500.degree. C. It was shown that high quality oxides can be grown at low temperatures without bird's beak. U.S. Pat. Nos. 4,323,589 and 4,232,057 to A. K. Ray and A. Reisman also describe this plasma assisted oxidation process.
The plasma assisted oxidation described in the above mentioned publications and patents uses a magnesium oxide (MgO) oxidation mask. Magnesium oxide is not commonly used in semiconductor fabrication processes because it is not readily available in semiconductor grade quality. Moreover, since magnesium oxide is not a conductor it cannot also be used as an electrode or contact. It must be removed after the plasma assisted oxidation. Unfortunately, magnesium oxide is difficult to remove using common etching processes.
It would be highly desirable to use more common materials as a mask in a low temperature plasma assisted oxidation, so that existing equipment and well known materials can be used. In fact, it would be most desirable to use polycrystalline silicon for such a mask because polycrystalline silicon is widely used in silicon semiconductor technology. Moreover, polycrystalline silicon may be appropriately doped to produce electrical contacts. However, if polysilicon is used as a mask in the above described plasma assisted oxidation process it will oxidize along with the monocrystalline silicon substrate that it is masking.